RNA polymerase gene, microorganism having said gene and the production of RNA polymerase by the use of said microorganism
This invention relates to a DNA that carries genetic information for the production of SP6 bacteriophage RNA polymerase, to new microorganisms (particularly new strains of Escherichia coli) harboring a plasmid into which said DNA has been integrated, and to a process for producing SP6 bacteriophage RNA polymerase by using said new microorganism. 
     SP6 bacteriophage RNA polymerase was discovered in 1982, and since then its biochemical properties have been investigated. With the recent progress in molecular genetics, there has been an increasing demand for processes to allow synthesis of uniform RNAs in large quantities and to prepare highly radioactive, single-stranded RNA probes to be used in Northern and Southern blotting techniques. Since RNA polymerase produced by bacteriophage SP6, which tends to infect Salmonella typhimurium, was found useful for this purpose, development of an advantageous process for manufacturing this RNA polymerase has been hoped for. 
     A method of Butler et al. is known for the manufacture of SP6 bacteriophage RNA polymerase, in which Salmonella typhimurium is subjected to multiple infection with SP6 bacteriophage and the RNA polymerase is recovered from the infected microbial cells [Journal of Biochemistry, 257, 5772 (1982)]. 
     However, the RNA polymerase cannot be obtained in large quantities by this method, because the amount of this enzyme contained in Salmonella typhimurium infected with bacteriophage SP6 is rather small and the operations involved are very intricate. On the other hand, nothing is known about isolation of the gene of SP6 bacteriophage RNA polymerase or about the method of cloning it after ligation with various vectors. 
     The object of this invention is to create new microorganisms (particularly new strains of Escherichia coli) harboring a plasmid that carries the SP6 bacteriophage RNA polymerase gene and suitable for industrial production of said RNA polymerase, and to provide a process for producing SP6 bacteriophage RNA polymerase by using said new microorganism. 
     Briefly, the first aspect of this invention relates to the gene of SP6 bacteriophage RNA polymerase. 
     The second aspect of this invention relates to new microorganisms harboring a plasmid into which SP6 bacteriophage RNA polymerase gene has been integrated. 
     The third aspect of this invention relates to a process for producing SP6 bacteriophage RNA polymerase, which comprises cultivating said new microorganism and recovering SP6 bacteriophage RNA polymerase from the culture broth. 
     We have succeeded in cloning 2.75 kb DNA fragment containing the whole region of SP6 bacteriophage RNA polymerase gene out of bacteriophage SP6 DNA, and found that cultivation of a microorganism, particularly a strain of Escherichia coli, into which a plasmid that carries said cloned DNA fragment has been integrated gives a significant quantity of SP6 bacteriophage RNA polymerase accumulated in the grown microbial cells. This invention was accomplished based on these findings. 
    
    
     This invention will be detailed below by referring to the accompanying drawings wherein: 
     FIG. 1 is a restriction enzyme cleavage map of the 2.75 kb DNA containing SP6 bacteriophage RNA polymerase gene; 
     FIG. 2 is the Hind III cleavage map of SP6 bacteriophage DNA; and 
     FIG. 3 shows the base sequence of the 2.75 kb DNA described later. 
    
    
     The new microorganism (for example, new strain of Escherichia coli) may be obtained according to the procedure given below. 
     (1) Phage DNA, extracted from bacteriophage SP6 (DNA donor), is cleaved with suitable restriction enzymes and a desired DNA fragment is recovered. 
     (2) The recovered DNA fragment is shortened by digestion at terminals with exonuclease Bal 31. 
     (3) A plasmid vector is cleaved with suitable restriction enzymes, and the DNA fragment obtained in (2) above is ligated to the cleaved ends of said plasmid. 
     (4) The plasmid into which the phage DNA fragment has been integrated is then introduced into a host, and a transformant that carries the intended DNA fragment is screened out. 
     (5) The plasmid is isolated from the transformant obtained in (4) above, and the desired DNA fragment is cut out from the isolated plasmid and ligated to an expression vector plasmid in the same manner as in (3) above. 
     (6) The expression vector plasmid obtained in (5) above (containing DNA fragment that carries SP6 bacteriophage RNA polymerase gene) is introduced into a host in the same manner as in (4) above. 
     The bacteriophage SP6 DNA used in the above process as DNA donor can be obtained by infecting Salmonella typhimurium LT2 strain with bacteriophage SP6, and recovering phage particles from the lysate thus formed, followed by extraction. Known techniques may be used for extraction, purification and cleavage with restriction enzymes, as detailed on pages 75-178 in &#34;Molecular Cloning, a Laboratory Manual&#34; (published from The Cold Spring Harbor Laboratory in 1982). 
     Cleavage of phage DNA with a restriction enzyme is carried out as described below. A suitable restriction enzyme is added to the reaction mixtures containing phage DNA, and cleavage reaction is carried out under suitable conditions to produce a number of DNA fragments. The enzyme used must be the one which is capable of cleaving phage DNA and does not cleave the DNA region that carries genetic information for the production of SP6 bacteriophage RNA polymerase. It is also preferable that the restriction enzyme cleaves the vector plasmid only at one site. 
     Cleavage of the plasmid vector is also effected in a similar way. 
     Known vector plasmids, for example pBR322, pUC18 and pUC19, may be used for the purpose of this invention. The above-mentioned DNA fragment is then spliced to the plasmid vector at its cleaved site by known techniques. The reaction conditions should be properly selected according to the types of vector DNA and restriction enzyme used. 
     An exonuclease is used in order to effect digestion at ends of DNA fragment and to give short DNA fragments. A known example is Bal 31 exonuclease, which produces DNA fragments of different lengths under various reaction conditions. 
     The plasmid carrying phage DNA fragment thus obtained is then introduced into a strain of Escherichia coli as host cells. Any type of Escherichia coli strain, both native and wild, may be used for this purpose so long as it is capable of transformation. It is also possible that a suitable type of Escherichia coli strain is selected depending on the type of vector plasmid used. 
     Cloning may then be effected depending upon the nature of plasmid vactor used, for example, by screening out ampicilin-resistant and tetracycline-sensitive colonies when pBR322 is employed as plasmid vector and the desired DNA fragment is ligated at its EcoRV site. 
     Analysis of cloned DNA thus obtained may be done by known methods; the length of cloned DNA fragment can be determined by isolating plasmids from many transformants obtained, followed by cleavage with a suitable restriction enzyme and electrophoresis on agarose gel. It was found that a plasmid carrying a cloned DNA with a length of 2.75 kb (pSP6-1) was actually obtained. 
     More detailed analysis revealed that this cloned DNA fragment carries SP6 bacteriophage RNA polymerase gene. FIG. 1 shows the restriction enzyme cleavage map of the 2.75 kb DNA carrying SP6 bacteriophage RNA polymerase gene. 
     The amount of RNA polymerase produced by the Escherichia coli strain into which plasmid pSP6-1 (pBR322 with SP6 bacteriophage RNA polymerase gene spliced thereto at its EcoRV site) has been introduced is not so large. It is therefore necessary to use other type of vector plasmid (for example, pUC18) in order to ensure mass production of RNA polymerase. When using pUC18, for example, plasmid pSP6-1 is treated with restriction enzymes, Bam HI and Hind III, to cut out DNA fragment of SP6 bacteriophage RNA polymerase, this fragment is spliced to pUC18 cleaved with Bam HI and Hind III, and the spliced molecule is used for transforming E. coli host cells (e.g., JM109 strain). Plasmid pSP6-2 was obtained in this way. 
     Whether an E. coli strain harboring a plasmid into which SP6 bacteriophage RNA polymerase gene has been integrated is producing the RNA polymerase protein or not may be examined by any known method, for example, by the method reported in Journal of Biochemistry, 257, 5772 (1982). When the protein is produced in significant quantities, it can be detected by lysing the microbial cells and subjecting the lysate thus obtained directly to SDS-polyacrylamide gel electrophoresis. 
     Plasmids carrying SP6 bacteriophage RNA polymerase gene, and E. coli strains harboring such a plasmid, can thus be prepared. 
     The following Examples will further illustrate the invention but are not intended to limit its scope. 
     EXAMPLE 1 
     (1) Preparation of bacteriophage SP6 DNA 
     Salmonella typhimurium LT2 strain was grown in 1 liter of modified L medium (containing 10 g/l Bacto. trypton, 5 g/l yeast extract, 5 g/l NaCl, 6 g/l disodium phosphate and 3 g/l monosodium phosphate) at 40° C. When the cell density reached 1.6×10 9  cells/ml, bacteriophage SP6 was added at a multiplicity of 0.05 pfu/cell, and the mixture was incubated at 40° C. until the microbial cells were lysed. 
     Chloroform (8 ml) was then added, the mixture was incubated for 15 minutes, DNase and RNase (1 mg each) were further added, and the resulting mixture was held at room temperature for 30 minutes. After addition of 29 g sodium chloride, the reaction mixture was allowed to stand at 4° C. for one hour and centrifuged. To 1 liter of the supernatant recovered, was added 60 g of polyethylene glycol #6000, the solution thus obtained was allowed to stand overnight at 4° C., and the resulting mixture was centrifuged, giving SP6 bacteriophage as precipitate. It was dissolved in 6 ml of B-A buffer [containing 0.5% Nonidet P40 (Shell Oil Co., Ltd.), 3.5 mM CaCl 2 , 5 mM MgCl 2 , 30 mM Tris-HCl (pH 7.5), 120 mM KCl, 0.5 mM EDTA and 30 mM 2-mercaptoethanol)]. The solution was overlaid on 3 ml of 40% glycerol buffer [ containing 40% glycerol, 0.5% Nonidet P40, 30 mM Tris-HCl (pH 7.5), 120 mM KCl and 30 mM 2-mercaptoethanol)] placed in a centrifuge tube, and subjected to ultracentrifugation at 4° C. (35000 rpm, 1 hour), thus recovering SP6 bacteriophage as precipitate. It was dissolved in 5 ml of Lysis buffer [containing 40 mM Tris-HCl (pH 8.0), 10 mM EDTA, 2% SDS and 100 μg/ml proteinase K], and the solution was held at 55° C. for one hour to complete the reaction. 
     To the reaction mixture thus obtained, was added an equal amount of a phenol solution [containing 10 mM Tris-HCl (pH 8.0) and phenol saturated with 1 mM EDTA], the mixture was gently stirred and then subjected to centrifugal separation, and the aqueous layer was collected (this operation is hereinafter referred to as &#34;phenol treatment&#34;). To this aqueous solution, was added twice its volume of ethanol, and the mixture was held at -70° C. for 30 minutes, thus giving, as precipitate, SP6 bacteriophage DNA (this operation is hereinafter referred to as &#34;ethanol precipitation&#34;). After removal of ethanol, the precipitate was dissolved in TE solution [containing 10 mM Tris-HCl (pH 8.0) and 1 mM EDTA)] to be ready for use in the next step. 
     (2) Preparation of restriction-enzyme cleavage map for SP6 bacteriophage DNA 
     Bacteriophage SP6 DNA (2 μg) obtained in (1) above was treated with 5 U of Hind III under optimum conditions [10 mM Tris-HCl (pH 7.5), 7 mM MgCl 2 , 60 mM NaCl] at 37° C. for one hour, and the reaction mixture was analyzed by electrophoresis on 1% agarose gel. The restriction enzyme cleavage map thus obtained is shown in FIG. 2, in which A˜N are DNA fragments cut out by Hind III. 
     (3) Analysis of the region similar to T7 RNA polymerase 
     The cleavage map shown in FIG. 2, the report by Kassavetis et al. [Journal of Biochemistry, 257, 5779 (1982)] and that by Stahl et al. [Journal of Molecular Biology, 148, 481 (1981)] suggested the possibility that information about RNA polymerase gene might be coded in the Hind III-C fragment (6.2 kb). Hence, it was attempted to determine part of the DNA sequence and convert it into amino acid sequence, thereby checking its similarity to T7 RNA polymerase. 
     SP6 DNA (100 μg) was cleaved with 300 U of Hind III. The resulting mixture was subjected to electrophoresis on 1% agarose gel, and the portion of gel containing 6.2 kb Hind III-C fragment was cut out. The cut out gel was put in a centrifuge tube and eluted into a buffer solution through electrophoresis, and the DNA was recovered from the eluate by addition of ethanol. 
     The DNA fragment thus obtained was then cleaved with restriction enzymes, Kpn I and Sau 3AI, and sequencing was done as described below according to the dideoxy method of Messing et al. [Method in Enzymology, 101, 20 (1983)]. The fragments obtained by celavage with Kpn I and Sau 3AI were spliced to M13 mp 18 vector at its Bam HI and Kpn sites, and the reconstituted molecule was used for transformation of E. coli JM109. The phage plaque thus obtained was grown in 1.5 ml of 2YT medium (containing 16 g/l Bacto. trypton, 8 g/l yeast extract and 5 g/l sodium chloride; pH 7.2), the single stranded DNA was recovered, and its sequence was determined (about 100 bp). After conversion into amino acid sequence, it was compared with that of T7 RNA polymerase, and high similarity was observed between the two. This indicates that the genetic information about SP6 bacteriophage RNA polymerase is coded in DNA fragment of about 2.75 kb containing the Kpn I site. 
     (4) Cloning of 2.75 kb DNA fragment containing SP6 bacteriophage RNA polymerase gene 
     Hind III-C DNA fragment obtained in (3) above (30 μg) was treated with restriction enzymes, Dra I (100 U) and Bbi II (100 U), in a buffer solution [Tris-HCl (pH 7.4), 5 mM HgCl 2 , 7 mM 2-mercaptoethanol and 0.01% BSA] at 30° C. for two hours to effect cleavage reaction. The DNA fragments thus obtained were separated by electrophoresis on 1% agarose gel, recovering 10 μg of 3.4 kb DNA fragment (D-B DNA fragment) in the same manner as in (3) above. 
     This D-B DNA fragment (5 μg) was then treated with 5 U of exonuclease Bal 31 in Bal buffer [20 mM Tris-HCl (pH 8.0), 12 mM CaCl 2 , 12 mM MgCl 2 , 1 mM EDTA and 600 mM NaCl] at 20° C. for five minutes. After the reaction was terminated by adding 10 μl of 0.5M EDTA, DNA (Bal DNA fragment) was recovered by phenol treatment, followed by ethanol precipitation. 
     pBR322 (2 μg) was used as the vector. It was treated with 5 U of restriction enzyme Eco RV in a buffer solution [10 mM Tris-HCl (pH 7.5), 7 mM MgCl 2 , 150 mM NaCl, 7 mM 2-mercaptoethanol and 0.01% BSA] at 37° C. for one hour, the enzyme was inactivated by heating the reaction mixture at 65° C. for ten minutes, and DNA was recovered by ethanol precipitation. It was dissolved in BAP buffer [100 mM Tris-HCl (pH 8.0)], 1 U of alkaline phosphatase was added, and the reaction was continued at 5° C. for 30 minutes. Vector DNA was recovered from the reaction mixture by phenol treatment, followed by ethanol precipitation. 
     Bal DNA fragment (0.1 μg) was ligated to pBR322 cleaved with Eco RV (0.15 μg) by treating the mixture of the two with 100 U of T4 DNA ligase in a buffer solution [Tris-HCl (pH 7.6), 6.6 mM MgCl 2 , 10 mM DTT and 0.5 mM ATP] at 16° C. for four hours. 
     Escherichia coli HB101 strain was grown in 5 ml of L medium (containing 10 g/l Bacto. trypton, 5 g/l yeast extract and 5 g/l sodium chloride; pH 7.2) at 37° C. for 16 hours. The master culture thus obtained (0.1 ml) was inoculated to 5 ml of fresh L medium, and shake culture was continued until OD (optical density) at 600 nm reached 0.6. The preculture thus obtained (2.5 ml) was ice-cooled and centrifuged at 3000 rpm for ten minutes. After complete removal of the supernatant, the residue was mixed well with 1.25 ml of ice-cooled 0.1M solution of CaCl 2 , and the resulting homogeneous mixture was held in ice water for 30 minutes and again subjected to centrifugation to remove the supernatant. To the residue thus separated, was added 10 μl of the DNA solution prepared above by ligation of Bal DNA fragment to pBR322, and the mixture was held in ice water for 30 minutes. It was then heated at 42° C. for 90 seconds, 2 ml of L medium was added, and the resulting mixture was held at 37° C. for 30 minutes. 
     The culture broth thus obtained was plated on L-agar medium containing 50 μg/ml ampicillin, and colonies grown at 37° C. were collected. These were replicated on L-agar medium comtaining 15 μg/ml tetracycline, those colonies which failed to grow were collected, and the plasmid involved was analyzed as described below. 
     The ampicilin-resistant and tetracycline-sensitive strain thus obtained was cultivated in 5 ml of L medium containing 50 μg/ml ampicillin at 37° C. for 16 hours, the grown cells were collected, 0.2 ml of Solution I [50 mM glucose, 10 mM Tris-HCl (pH 8.0), 5 mM EDTA and 2 mg/ml lysozyme)] was added, and the mixture was held in ice water for 30 minutes. Then added was 0.4 ml of Solution II (0.2N NaOH and 1% SDS), followed by addition of 0.3 ml Solution III [3M sodium acetate (pH 4.8)] five minutes later, and the mixture was held at 0° C. for 30 minutes and cenrtrifuged. The supernatant was collected, ethanol was added, the precipitate which separated out was dissolved in 0.2 ml of Solution IV [25 mM Tris-HCl (pH 8.5), 1 mM EDTA, 150 mM NaCl and 1 mg/ml RNase A], and the resulting solution was held at 37° C. for 30 minutes. At the end of reaction, 0.2 ml of Solution V (20% polyethylene glycol #6000 and 2M NaCl) was added, and the mixture was held at -20° C. for 30 minutes. 
     The precipitate, collected by centrifugation, was dissolved in TE solution, and plasmid DNAs were recovered by phenol treatment and ethanol precipitation and dissolved in 10 μl of TE solution. 
     This DNA solution was treated with Bam HI and Hind III, and the sizes of DNA fragments integrated were determined by electrophoresis on agarose gel. It was found that plasmids of various sizes had been obtained (the longest DNA contained being 2.75 kb). The plasmid which contains this 2.75 kb DNA was named pSP6-1. The base sequence of the 2.75 kb DNA is shown in FIG. 3. The Escherichia coli strain carrying said plasmid, Escherichia coli HB101/pSP6-1, has been deposited at Fermentation Research Institute, the Agency of Industrial Science and Technology of Japan, under FERM BP-1418. This is a plasmid carrying the whole region of SP6 bacteriophage RNA polymerase gene. 
     (5) Ligation of DNA fragment carrying SP6 bacteriophage RNA polymerase gene to pUC18 and introducing the resulting plasmid into Escherichia coli 
     Palsmid pSP6-1 DNA obtained in (5) above was cleaved with Bam HI and Hind III, and the reaction mixture was subjected to agarose gel electrophoresis, giving 3.1 kbp DNA fragment containing the above-mentioned 2.75 kb DNA. Separately, pUC18 DNA was also cleaved with Bam HI and Hind III, and the resulting mixture was treated with alkaline phosphatase in the same maner as in (4) above, giving the intended vector DNA. 
     The two DNA fragments obtained above were linked together in the same way as above, and the reconstituted molecule was used for transforming Escherichia coli JM109 strain (Gene, 33, 103-119, 1985). Analysis of the plasmid contained in the transformant in the same way as in (4) above showed that it contains 3.1 kb DNA fragment, and this plasmid was named pSP6-2. 
     (6) Production of SP6 bacteriophage RNA polymerase by Escherichia coli strain carrying plasmid pSP6-2 (Escherichia coli JM109/pSP6-2) 
     The above-mentioned E. coli strain was inoculated to 10 ml of L medium containing 50 μg/ml ampicillin and cultivated at 37° C. for 16 hours. The preculture thus obtained was transferred to 500 ml of L medium placed in a 2-liter flask, and the cells were cultivated at 37° C. for five hours (120 rpm). After adition of 50 mg/ml IPTG (isopropyl-1-thio-β-D-galactopyranoside), cultivation was continued for an additional three hours, and the grown cells were collected and suspended in 15 ml of buffer solution IV [50 mM Tris-HCl (pH 8.0), 10% sucrose and 10 mM 2-mercaptoethanol]. To this suspension, were added 20 μg/ml of PMSF (phenylmethanesulfonyl fluoride) and 2 mg/ml of lysozyme, and the mixture was held in ice water for 30 minutes. 
     Spermidine (0.12 g) and deoxycholic acid (to a concentration of 0.05%) were then added, and the resulting mixture was stirred for five minutes and subjected to ultracentrifugation (10500×g, 1 hour) to recover the supernatant. Activity measurement revealed 800,000 U SP6 bacteriophage RNA polymerase produced--approximately 30 times higher productivity than the case when Salmonella typhimurium LT2 strain was infected with SP6. 
     As is apparent from the foregoing, SP6 bacteriophage RMA polymerase gene was first isolated in this invention. It was also demonstrated that microorganisms containing a plasmid that carries said gene are capable of effectively producing SP6 bacteriophage RNA polymerase which is of great use in genetic engineering. 
     The meanings of the various symbols used in FIG. 3 are well known in the art of genetic engineering technology and are defined, for example, in &#34;Chemistry and Biochemistry of the Amino Acids&#34;, page 9, Edited by G. C. Barrett, published by Chapman and Hall, 1985.